Micromachined pellicle splitters and tunable laser modules incorporating same

ABSTRACT

A micromachined pellicle beam splitter and method of manufacture thereof are disclosed. In one embodiment, the beam splitter includes a silicon frame with a silicon nitride membrane attached to the frame and covering an opening through the frame. Other materials may be utilized, however, the coefficient of thermal expansion (CTE) of the membrane should be greater than that of the frame. The beam splitter may be manufactured by coating a silicon substrate with a layer of silicon nitride, patterning an opposite side of the silicon substrate with a photoresist or a metallic layer to define an opening an etching an opening through the substrate to the silicon nitride with either a dry etch or wet etch technique. An improved tunable laser module incorporating the micromachined pellicle beam splitter and a method of tuning a laser diode are also disclosed.

TECHNICAL FIELD

Micromachined pellicle optical beam splitters are disclosed. Morespecifically, pellicle beam splitters are disclosed which comprise asilicon frame and a silicon nitride membrane. Methods of manufacturingthe disclosed beam splitters using wet and dry etch techniques arc alsodisclosed. Tunable laser modules including a disclosed pellicle beamsplitter are also disclosed.

BACKGROUND OF THE RELATED ART

Pellicle bean splitters are known. Currently available pellicle beamsplitters are relatively large in size and consist of a nitrocellulosemembrane or pellicle stretched over a rigid frame. The frames are oftenfabricated from metal, such as aluminum.

A light source is directed at the membrane and a known fraction of theoptical amplitude is reflected while a majority of the optical amplitudeis transmitted through the membrane. Pellicle beam splitters are usefulin monitoring the amplitude of the light transmitted through the beamsplitter. The known fraction of the optical amplitude that is reflectedcan be transmitted to a monitor photodiode where a determination can bemade as to whether an adjustment to the optical amplitude is necessary.

As noted above, optical beam splitters are relatively large in size andcannot be used in smaller applications such as telecommunication modulesand other applications that use semiconductor lasers as the lightsource. Accordingly, there is a need for a beam splitter that is aseffective as a pellicle beam splitter in transmitting a majority of theoptical amplitude while reflecting a known fraction of the amplitude formonitoring purposes and that further is small enough for thetelecommunication modules and other laser applications.

There is an increasing demand for tunable lasers given the advent ofwavelength-division multilplexing (WDM) which has become widespread infiber optic communication systems. WDM transponders include a laser, amodulator, a receiver and associated electronics. One WDM transponderoperates a fixed laser in the near-infrared spectrum at around 1550 nm.A 176 wavelength system uses one laser per wavelength and therefore sucha system typically must store a 176 additional WDM transponders asspares to deal with failures. This high inventory requirementcontributes to the high cost of these systems.

In response, tunable lasers have been developed. A single tunable lasercan serve as a back-up for multiple channels or wavelengths so thatfewer WDM transponders need to be stocked for spare part purposes.Tunable lasers can also provide flexibility at multiplexing locations,where wavelengths can be added and dropped from fibers as needed.Accordingly, tunable lasers can help carriers effectively managewavelengths throughout a fiber optics network.

Two currently available tunable lasers are distributed feedback (DFB)lasers and distributed brag reflector (DBR) lasers. A conventionaltunable laser module 10 is illustrated in FIG. 1. In tunable lasers, theoutput power is most often measured from the front of the laser diodegain chip 12 of the laser 11, and not from a rear facet as is done withnon-tunable lasers. The output of the laser diode gain chip 12 isdirected through a collimating lens 13 and isolator 14. The opticaloutput then engages the cubicle power tap 15 at an angle of about 45°where a fraction of the light is reflected toward a detector shown at 16and the remaining output passes through the lens 17 to the fiber 18. Thedetector 16 and diode gain chip 12 are linked by various circuitry shownat 19 for tuning the laser or laser diode shown at 12.

A cube power tap 15 is typically a solid, coated optical elementassembled into a standard beam splitter cube that reflects a smallportion of the light and sends it to the detector 16 as discussed above.However, one difficulty with the standard beam splitter cube 15 is thatit has many surfaces that can provide stray reflections. Although theamplitude of the stray reflections may be relatively small due toantireflection coatings applied to the surfaces of the cube 15, thepresence of the reflected light can interfere with small signals thatare typical of servo signal inputs used by the control mechanism 19 anddiode gain chip 12 to adjust the wavelength of the laser 12.

As a result, there is a need for an improved power tap device which caneliminate the stray reflective rays.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed apparatuses and methods are illustrated more or lessdiagrammatically in the accompanying drawing wherein:

FIG. 1 is a schematic illustration of a tunable DFB or DBR laser modulein accordance with the prior art;

FIG. 2 is a sectional view of a micromachined pellicle beam splittermade in accordance with this disclosure;

FIG. 3 illustrates, graphically, the reflectivity of a silicon nitridefilm versus film thickness for a P polarization, C band light wavedirected at a silicon nitride film at a 45° angle of incidence;

FIG. 4 illustrates, graphically, the spectral performance of fivepellicle membranes set in P polarization at a 45° angle of incidencewherein the membranes have thicknesses of about 25 nm, 426 nm, 827 nm,1228 nm and 1529 nm.

FIG. 5 is a schematic illustration of a tunable laser moduleincorporating a disclosed micromachined pellicle beam splitter as shownin FIG. 1;

FIG. 6 illustrates, graphically, detector sensitivities orresponsivities versus wavelength for InGaAs, Ge and Si detectors thatcan be used to design an appropriate micromachined pellicle beamsplitter for a tunable laser module incorporating one of said detectors;

FIG. 7 illustrates, graphically, the spectral performance of a siliconnitride pellicle film set in P polarization at a 45° angle, at near zerowave solution, for three membranes, all at about a half wavelengththickness for near zero wave solution, i.e.; about 34, 30 and 26 nm orabout 30 nm and +/−12%;

FIG. 8 illustrates, graphically, the spectral performance of threepellicle membranes set in P polarization at a 45° angle wherein onemembrane has a thickness less than a half wavelength for the C band(˜370 nm) and the two other films shown are about 2% thicker and about2% thinner than the initial films, i.e., about 362 and 378 nm; and

FIG. 9 illustrates, graphically, the spectral performance of threepellicle membranes set in P polarization at a 45° angle wherein thefirst membrane has a thickness greater than a half wave for the C band(˜430 nm) and the other two membranes have thicknesses that are about 2%greater and 2% thinner than the first membrane, i.e., about 438 and 422nm.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A silicon micromachined pellicle beam splitter is disclosed. As shown inFIG. 2, a beam splitter 30 includes a silicon frame 31 that is coatedwith a silicon nitride membrane 32. The frame 31 is fabricated from asilicon substrate using dry or wet etch processes. For example, if adraft angle is desired as indicated by the tapered wall 33 shown inphantom in FIG. 2, a wet etch process may be required. If no draft angleis desired, then a dry etch process can be used.

The substrate 31 is coated with the silicon nitride layer 32. Then, aphotoresist, metal or other protective layer (not shown) is coated ontothe underside 34 of the substrate 31 leaving an uncoated area thateventually defines the etched volume shown at 35. An etch process iscarried out to create the etch volume 35 without damaging the siliconnitride layer 32. If a wet etch process is utilized, potassium hydroxideis a suitable etchant.

It may also be desirable to include a protective support shown inphantom in FIG. 2 at 36. If this is the case, then the silicon nitridefilm 32 is sandwiched between the silicon substrate 31 that becomes theframe 31 and an additional silicon substrate 36. Again, anotherprotective layer such as a photoresist or metallic layer is coated ontothe top side 37 of the substrate 36 and the etching process is carriedout through the substrate 31 and through the substrate 36 leaving thesilicon nitride membrane 32 intact. If a draft angle is desired for thesubstrate or frame 31, a wet etch process may be carried out through thesubstrate 31 and if no draft angle is warranted for the protective frame36, a dry etch process may be carried out for the substrate 36.

If a draft angle is desired for one substrate 31 but not the othersubstrate 36, or 25 vice versa, then the wet and dry etchings arecarried out separately. Otherwise, if the same etching technique is usedfor both substrates 31, 36, the etchings may be carried outconcurrently.

For tunable laser applications, the thickness of the silicon nitridefilm or pellicle 32 should be on the order of about 20-60 nm becausesuch a thickness results in a reflectivity of about 1% in the Ppolarization in the C band at a 45° angle of incidence as shown in FIG.3. This approximately 1% reflectivity is a convenient level for powermonitoring.

Further, thicknesses for the membrane 32 of approximately one-half ofthe optical wave for C band light can also be achieved. As shown in FIG.3, in addition to low reflectivities for thin silicon nitride films withthickness less than 60 nm, low reflectivities are also exhibited forsilicon nitride films having thicknesses of about one-half of theoptical wave for C band light. Films of these thicknesses may alsoprovide low reflectivity at the design wavelength. It may be convenientfor a power monitoring application that the calibration curve neverencounters a zero in reflectivity. Thus, it may be desirable for thethickness of the membrane 32 of the pellicle beam splitter 30 to be moreor less than one half of a wavelength thickness optically.

FIG. 4 illustrates, graphically, the reflectivity that results fromvarious selected silicon nitride film thicknesses, 25, 426, 827, 1228and 1629 nm, as a function of wavelength. It will be noted that thereflectivity at a 1550 mm wavelength using a thin, 25 nm film thicknessremains relatively constant. Therefore, thin silicon nitride films(20-40 nm) may prove to be more convenient for the wavelength rangeshown in FIG. 4 because of the constant reflectivity or relatively flatslopes of the reflectivity curves.

Further, more complex film stacks may be utilized depending upon thespectral property desired. Thus, FIG. 2 also shows an optional layer 38may be used to protect the silicon nitride layer 32 or vary the spectralproperty of the beam splitter 30. One suitable material for theadditional layer 38 is silicon dioxide. However, other materials will beapparent to those skilled in the art who desire to vary the spectralproperties of the beam splitter 30. Film stacks of three or more filmsare contemplated and may be desirable for a variety of applications.

The combination of silicon for the substrate or frame 31 and siliconnitride for the membrane 32 is advantageous because silicon has acoefficient of thermal expansion on the order of about 2.6 while siliconnitride has a coefficient of thermal expansion on the order of about 4.As a result, the silicon nitride membrane 42 will remain in a state oftension which results in the low reflectivity of the beam splitter 30.Because silicon dioxide has a CTE of about 0.5, it would not a suitablematerial for the membrane layer 38 when silicon is used for the frame31. Materials other than silicon nitride could be used for the membranelayer 32, however, the coefficient of thermal expansion of the membranelayer 32 should be greater than that of the material used for thesubstrate or frame 31.

The draft angle provided by the wall shown in phantom at 33 in FIG. 2 isuseful if an angle of incidence of about 45° is utilized. The draftangle provided by the wall 33 reduces the amount of clipping caused bythe frame 31.

Another advantage to the beam splitter 30 is the very small beamdisplacement upon transmission. Specifically, the amount of the beamdisplacement is less than the thickness of the membrane layer 32 and, asa result, the use of very small beams with the beam splitter 30 ispossible and therefore the beam splitter 30 will be useful in telecommodules and other devices requiring the use of very small beams.

While an approximately 30 nm thickness has been suggested for themembrane layer 32, particularly if silicon nitride is chosen as thematerial for the membrane 32, the 30 nm thickness is suggested for smallbeam applications, such as telecom modules. The thickness of themembrane 32 can vary greatly, depending upon the particular application.The use of a thin film, however, permits a wide range of convergencewith minimal affect on interference properties. Further, thin films aretypically very parallel, which avoids substantial angular displacementof the beam upon transmission through the beam splitter 30.

FIG. 5 illustrates a tunable laser module 10 a equipped with a pelliclebeam splitter 30 as disclosed in FIG. 2. The components of the module 10a that are the same as those shown in FIG. 1 will be referenced withlike reference numerals with the suffix “a”. For the reasons set forthabove, the beam splitter 30 is superior to the cube 15 (FIG. 1) becauseof its ability to eliminate stray reflections.

Specifically, the components of the laser 11 a include a back cavitymirror 22 with a reflective coating. Between the diode gain chip 12 aand the back cavity mirror 22 are one or more thermally tuned filtersshown at 20, 21 and a diode intracavity collimating lens 25 or lasercavity lens 25. Light reflected off of the back cavity mirror 22 passesthrough the filters 20, 21 and through the lens before passing throughthe diode gain chip 12 a where it again passes through a diode outputcollimating lens 13 a before passing through the isolator 14 a to thepellicle beam splitter 30. A small fraction of light is reflected off ofthe pellicle beam splitter 30, which as shown in FIG. 5 is preferablydisposed at an angle of about 45° to the light path. The small fractionof light is detected at the detector 16 a and a signal is transmitted tothe adjustment circuitries shown at 19 a for tuning the laser diode gainchip 12 a. The majority of the light passes through the pellicle beamsplitter 30 and through the fiber focusing lens 17 a to the polarizationpreserving fiber 18 a

FIGS. 4 and 6-9 illustrate various methodologies for designing themicromachined pellicle beam splitter 30 for use in a tunable lasermodule 10 a.

Referring to FIG. 6, it is well known that the responsivities of thedetector shown at 15 a will vary depending upon the wavelength detectedand therefore the tuning range of the module 10 a. Variances in detectorsensitivity can decrease the effective resolution of the detectorcircuitry (16 a, 19 a and 12 a) and may require the use of extensivelook-up tables for power calibration.

As shown in FIG. 6, silicon detectors in the visible, germanium in theS-band, C-band and L-band and indium/gallium/arsenic detectors in theL-band all exhibit variations and detectors sensitivity acrossrelatively wide tuning ranges. Design of the pellicle beam splitter 30incorporated into a tunable laser module such as that shown at 10 a inFIG. 5 can compensate for at least some variations in detectorsensitivity as illustrated in FIGS. 7-10.

Turning to FIG. 7, the spectral performance of three pellicle membranesof a disclosed beam splitter 30 is shown where the line 41 represents asilicon nitride film having a thickness of 34 nm, the line 42 representsa silicon nitride film having a thickness of about 30 nm and the line 43represents silicon nitride film having a thickness of about 26 nm. Thesethree films represent thicknesses approaching a near zero wave solutionas shown in the graph of FIG. 3. The film represented by the line 41 isapproximately 12% thicker than the film represented by the line 42 andthe film represented by the line 43 is about 12% thinner than the filmrepresented by the line 42. The response of these silicon nitride filmsis nearly flat over the wavelength range of interest. Hence, choosingthese thicknesses will provide little or no compensation for thegermanium or indium/gallium/arsenic detectors illustrated in FIG. 6.However, for the flat portion of the indium/gallium/arsenic curve (1500to 1600 nm), the thin silicon nitride membranes would be suitable. Thesemembranes would not provide a compensating effect for a germaniumdetector over the same wavelength and thus, a different compensationscheme, if desired, would need to be investigated as shown below.

Turning to FIG. 8, the spectral performance of a pellicle membrane 32 ofa beam splitter 30 set in P polarization at a 45° angle is presented.The line 44 represents a silicon nitride film having a thickness ofabout 370 nm, the line 45 represents a silicon nitride film having athickness of about 362 nm and the line 46 represents silicon nitridefilm having a thickness of about 378 nm. As can been seen in FIG. 8, thereflectivity increases as wavelength increases for these three filmswhich would be useful in compensating for the drop in responsivity of agermanium detector at wavelengths exceeding 1500 nm or in the C-band. Inother words, using the disclosed pellicle beam splitters, with anappropriate silicon nitride thickness, can greatly assist in flatteningout the responsivity curve for a germanium detector and the C-band (seeFIG. 6). Thus, referring to FIGS. 8 and 3 together, films havingwavelengths approaching the half-way thicknesses shown in FIG. 3 (i.e.,300 to 390 nm, 700 to 790 nm or 1100 to 1190 nm) would prove useful inflattening out the responsivity curve for a germanium detector in theC-band.

Turning to FIG. 9, the spectral performance of a pellicle beam splitter30 with a silicon nitride membrane 32 is illustrated in P polarizationat a 45° angle of incidence wherein the thicknesses of the siliconnitride membrane 32 are longer than a C-band wavelength. Specifically,the line 47 represents a membrane 32 with a thickness of about 430 nm,the line 48 represents a silicon nitride membrane with a thickness ofabout 438 nm and the line 49 represents a silicon nitride membrane 32with a thickness of about 422 nm. Thus, the membranes represented by thelines 48 and 49 are slightly thicker and thinner (+/−2%) than themembrane represented by the line 47. As can be seen in FIG. 9, thereflectivity decreases as the wavelength increases. These membranescould be useful for an indium/gallium arsenic detector in the C-band orthe S-band range to compensate for the downward slope of theresponsivity curve for an indium/gallium/arsenic detector as thewavelength increases (see FIG. 6). Referring to FIGS. 3 and 9, membraneshaving thicknesses greater than the half-way thickness for the C-band asshown in FIG. 3, i.e., 10 to 80 nm, 410 to 480 nm, 810 to 880 nm, 1210to 1280 nm or 1610 to 1680 nm would prove useful for flattening out thedownward slope of the responsivity curve for an indium/gallium/arsenicdetector as shown in FIG. 6.

Similarly, returning to FIG. 4, the spectral performance of 5 siliconnitride membranes 32 set in P polarization at a 45° angle of incidenceis presented wherein the line 51 represents a membrane having athickness of about 25 nm, the line 52 represents a membrane having athickness of about 426 nm, the line 53 represents a membrane having athickness of about 827 nm, the line 54 represents a membrane having athickness of about 1228 nm and the line 55 represents a membrane havinga thickness of about 1629 nm. The downward slope of these lines aswavelength increases could be used to compensate for the upward slope ofthe responsivity curve of an indium/gallium/arsenic detector in theC-band range or the S-band range (see FIG. 6). Referring to FIGS. 9 and3, membranes having thicknesses at or close to the half-wave wavelengthsshown in FIG. 3, i.e., 10 to 50 nm, 410 to 450 nm, 810 to 850 nm, 1210to 1250 nm and 1610 to 1650 nm would be useful in compensating for theupward slope of the responsivity curve of an indium/gallium/arsenicdetector in the C-band range r the S-band range as shown in FIG. 6.

Thus, an improved tunable laser module. 10 a is disclosed whereby apellicle beam splitter 30 as disclosed herein, with an appropriatelyselected silicon nitride membrane 32 thickness that can compensate forvariances in responsivity of the detector 16 a over the tunablewavelength range.

Specifically, referring to FIG. 5, light is generated by the laser 11 aand reflected off of the back cavity mirror 22, through one or morefilters shown at 20, 21 and through the diode intracavity collimatinglens 25 to the diode gain chip 12 a. Light emerges from the diode gainchip (or other suitable gain media) 12 a and passes through anothercollimating lens 13 a before passing through an isolator 14 a. Lightemerging from the isolator 14 a engages the pellicle beam splitter 30where a fraction is reflected to the detector 16 a and an adjustment tothe diode gain chip 12 a output is made either directly or by way of acontrol circuitry 19 a. Thus, light passing through the beam splitter 30and through the fiber focusing lens 17 a to the polarization preservingfiber 18 a is constantly monitored by way of the beam splitter 30 anddetector 16 a and tuned or adjusted by way of the circuitry 19 a anddiode gain chip 12 a are other suitable gain media Various other controlloops will be apparent to those skilled in the art. Thus, the wavelengthof the output from the laser 11 a can be adjusted by modifications tothe one or more thermally tuned filters shown at 20, 21.

In the foregoing detailed description, the disclosed structures andmanufacturing methods have been described with reference exemplaryembodiments. It will, however, be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope of this disclosure. The above specification and figuresaccordingly are to be regarded as illustrated rather than restrictive.Particular materials selected herein can be easily substituted for othermaterials that will be apparent to those skilled in the art and wouldnevertheless remain equivalent embodiments of the disclosed devices andmanufacturing methods.

1. A pellicle beam splitter comprising: a frame comprising an openingextending there through, the frame comprising a first material having afirst coefficient of thermal expansion (CTE), a membrane comprising afilm attached to the frame and covering the opening, the membranecomprising a second material having a second CTE, the first CTE beingless than the second CTE, and a protective frame with an opening inmatching registry with the opening of the frame and being disposed overthe membrane to sandwich the membrane between the protective frame andthe frame.
 2. The beam splitter of claim 1 wherein the first material issilicon and the second material is silicon nitride.
 3. The beam splitterof claim 1 wherein the membrane is in a state of tension.
 4. The beamsplitter of claim 1 wherein the frame comprises a first side and asecond side, the opening extending from the first side to the secondside, the membrane being attached to the second side, the opening beingwider at the first side of the frame and narrower at the second side ofthe frame to provide a draft angle.
 5. The beam splitter of claim 1wherein the first material is in a crystallized state and the opening isproduced by a wet etch process.
 6. The beam splitter of claim 1 whereinthe first material is silicon, the second material is silicon nitrideand the protective frame is fabricated from silicon.
 7. The beamsplitter of claim 1 further comprising a second thin film disposed overthe membrane and sandwiching the membrane between the frame and thesecond film.
 8. The beam splitter of claim 1 further comprising aplurality of films stacked on top of the membrane.
 9. A pellicle beamsplitter comprising: a crystalline frame comprising an etched openingextending there through, a membrane comprising a film attached to theframe and covering the opening, the membrane being in a state oftension, and a protective frame with an etched opening in matchingregistry with the etched opening of the frame and being disposed overthe membrane to sandwich the membrane between the protective frame andthe frame.
 10. The beam splitter of claim 9 wherein the frame comprisinga first material having a first coefficient of thermal expansion (CTE),the membrane comprising a second material having a second CTE, the firstCTE being less than the second CTE.
 11. The beam splitter of claim 10wherein the first material is silicon and the second material is siliconnitride.
 12. The beam splitter of claim 9 wherein the frame comprises afirst side and a second side, the opening extending from the first sideto the second side, the membrane being attached to the second side, theopening being wider at the first side of the flame and narrower at thesecond side of the frame to provide a draft angle.
 13. The beam splitterof claim 9 wherein the first material is silicon, the second material issilicon nitride and the protective frame is fabricated from silicon andthe etching of the silicon frames is carried out with a wet etchprocess.
 14. The beam splitter of claim 9 further comprising a secondthin film disposed over the membrane and sandwiching the membranebetween the frame and the second film.
 15. The beam splitter of claim 9further comprising a plurality of films stacked on top of the membrane.